1. Field of the Invention
The present invention relates to a process for the transposition of an optical modulation having one wavelength to another adjustable wavelength. It is used in optical telecommunications.
2. Discussion of the Background
The introduction of optics into telecommunications networks has been due to the great transmission capacity offered by optical links. In order to take full advantage of this and obtain an optical transparency over the entire network, it is necessary to limit to the maximum the use of optical/electrical and electrical/optical connections. This is the reason why optical amplifiers (with doped fibres or semiconductors) have been developed. By ensuring an all-optical regeneration of the signals, they make it possible to eliminate electrical repeaters.
In a telecommunications network using several wavelengths (so-called multicolour network), the routing operation (known as space-division switching of circuits or wavelength switching) involves basic functions fulfilled by wavelength-tunable transmitters, large capacity star couplers or tunable optical filters. To these functions must be added that of wavelength transposition consisting of transferring the information from one optical carrier to another. It permits not only the switching of an information in a network by "colouring" the signal, but also the connection between two local multicolour networks.
In order to obtain this transposition, one possibility consists of detecting the information received on one wavelength and using the resulting electrical signal for modulating a laser emitting at another wavelength.
A device able to fulfil this function can be in two different forms:
in the first, the information is initially on a fixed wavelength .lambda..sub.o and is transposed to another wavelength .lambda..sub.i chosen from among n possible wavelengths, so that it is successively possible to access, at the output of the device, the information on any of the wavelengths .lambda..sub.1, .lambda..sub.2, . . . , .lambda..sub.n ;
in the second, the information reaches the device on a wavelength .lambda..sub.i selected from among n possible wavelengths and the device transfers the signal to another fixed wavelength .lambda..sub.o.
Hitherto, this transposition operation has been obtained with the aid of laser diodes and semiconductor amplifiers. In the first case, action occurs on the transmission or emission wavelength of a monomode laser of the distributed feedback or DFB type or distributed Bragg reflection or DBR type, by optical injection in the active zone.
This is described in the two following articles:
P. POTTIER, M. J. CHAWKI, R. AUFFRET, G. CLAVEAU, A. TROMEUR, "1.5 Gbit/s transmission system using all optical wavelength convertor based on tunable two-electrode DFB laser", Electron. Lett., vol. 27, pp 2183-2184, 1991;
B. MIKKELSEN, T. DURHUUS, R. J. PEDERSEN, K. E. STUBKJAER, "Penalty free wavelength conversion of 2.5 Gbit/s signals using a tunable DBR-laser", Proc. ECOC'92, Berlin, September 1992.
With regards to semiconductor amplifiers, they make use either of the gain saturation variation phenomenon, or mixing with four waves, in order to bring about the transposition.
This is described in the two following articles:
J. M. WIESENFELD, B. GLANCE, "Cascadability and fanout of semiconductor optical amplifier wavelength shifter" IEEE Photon. Technol. Lett., vol. 4, pp 1168-1171, 1992;
G. GROSSKOPF, R. LUDWIG, H. G. WEBER, "140 Mbit/s DPSK transmission using an all-optical frequency convertor with a 4000 GHz conversion range" Electron Lett., vol. 24, pp 1106-1107, 1988.
Despite their interesting performance characteristics, these different processes have a high level of complexity, namely either the process is difficult to perform (particularly with regards to the adjustments of the wavelengths of the incident and transpose signal) or require the use of a pumping laser.